[erlang-questions] gen_server state is not released for gc

[Björn-Egil Dahlberg]

Den 13 januari 2012 23:11 skrev Max Lapshin <max.lapshin@REDACTED>:

> I thought, that calling gc will be a problem:
>
>
> A = crypto:rand_bytes(1024*1024*1024),
> erlang:garbagecollect(),
> {reply, ok, State}
>
>
> Will gc free A? I think that no, because A is still used in the place,
> where gc is called. Am I right?
>

Correct.

To be a little technical (and don't take this as an absolute truth in the
details),
What we see here is "A is referencing a large binary", which is true. What
happens internally is that Eterm A is a boxed pointer from the stack or a
register which points to a ProcBin on the heap. The ProcBin in turn holds
an increment of a Reference counted binary off heap. Both the stack and the
registers will become part of the root set during the garbage collection
hence letting the binary be reachable and therefore "live".  I believe that
you need leave the function (release the stack) in order to free your
binary.

A gc without the A in the stack will not let the ProcBin survive during the
copy phase of the gc. When the ProcBin is removed the RefcBinary will
decrement and if it reaches zero, i.e. the last ProcBin has died, that too
will be removed.

// Björn-Egil
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